A radio wave in dozens of gigahertz of band which is called as millimeter wave band has high path loss, short wavelength and high straightness. Accordingly, a directional beamforming technique is introduced as a useful method for enhancement of capacity and coverage, wherein the directional beamforming technique can use effectively line of sight LOS or multipath component. An analog beamforming technique ABF has been mainly used compared with a digital beamforming technique because the analog beamforming technique is less complicated than the digital beamforming technique in view of hardware. Additionally, a switched beamforming technique has been widely used than an adaptive beamforming technique due to constraint of the ABF.
It is possible to realize a small array antenna system including many antennas in millimeter wave band because wavelength of the millimeter wave is short. In this case, the array antenna system can form many beams of which half power beam width HPBW is very small. Additionally, the array antenna system can achieve three-dimensional beamforming, which realizes both of vertical beam and horizontal beam, by using hundreds of antennas. The array antenna system can obtain considerable high gain by arraying a beam in transmitter side and a beam in receiver side, i.e. a pair of beams to realize maximum signal to noise ratio SNR.
Accordingly, it is necessary to search the optimal beams in transmitter side and receiver side (beam training) so as to achieve maximum beamforming performance. Since pairs of beams exist in multi path, it is important to search a pair of beams capable of realizing maximum beamforming performance. The beam training is generally achieved by repeating corresponding process, and thus a time proportional to multiply of a number of a beam in a transmission station and a number of a beam in a receiving station is approximately required. Specially, in the event that a lot of beams exist, much amount of time is taken for realizing the beam training. Hence, a technique for achieving the beam training in short time has been required, for the purpose of performing effective beamforming.